from the outlet tester indicates that the isolation transformer may not be doing its job. The fact that the outlet tester doesn't show that neutral is disconnected from ground seems like a problem, but perhaps there is enough leakage/capacitive coupling to fool it.

To double-check the outlet tester, put about a resistor of somewhere around 100K to 470K in parallel with the test leads of your Fluke 77, and re-measure the voltages between neutral and ground and hot and ground. If they are not something very close to zero, you've got a problem.

May we see a photo of the isolation transformer wiring? Perhaps there is some mistake that grounds one end of the secondary.

Thanks Michael - could you explain further? I've checked for continuity from every terminal on the variac to the cabinet and found none. I think the cabinet should be grounded and I've done so with the green wire on the power cord, which then grounds the outlet through the mounting screw. I think this should be ok since the isolation transformer is supposed to break the connection between the ground wire and the neutral wire. What am I missing?

Hello again Rich. There are only a few possibilities here in order for the iso to have any relationship to earth ground, mains hot, or mains neutral. Eliminating the mains ground is a start, but you've already done that. (House wiring "grounds" aren't always reliable. I've seen instances when the ground is actually carrying current, due to mis-wiring somewhere in the house, or a defective device plugged in somewhere in the house. Mis-wired ceiling fans and 3-way switches are often culprits.) Verifying that your power switch isn't defective is another possibility. You've checked the all the variac terminals to ground/mounting hardware, and you've also done that with the iso. The only other possibility is that the internal wiring of the iso is misconnected, or was wired for another purpose earlier in its's life (perhaps being used in a buck/booster circuit comes to mind). There really are no other possibilities . . . . Actually, I'd suspect the transformer . . .

I think the cabinet should be grounded and I've done so with the green wire on the power cord, which then grounds the outlet through the mounting screw.

The cabinet should be grounded. The outlet should not. The transformer does not have anything itself to do with the ground, but when you connect it to the outlet, you are now bridging the sides, kind of negating what the transformer is there for.

What I was saying before was on a standard issue outlet, the metal frame of the outlet is connected to the ground slot of the outlet. So if you have it's metal frame mounted right to the metal case, which by the way has the ground screw connected to it, the outlet now ends up grounded.

Why? Give me a good reason. As Leigh pointed out, anything that needs to be isolated will have a 2 wire cord so the ground isn't connected anyway. Grounding the outlet shouldn't cause any problem with making the unit work or in it's future use. Unless, of course, the outlet itself is defective.

I propose that the meter and the outlet tester both draw too little current to give a reliable test. As Leigh says, put a 10K 2W (not 220K) resistor in parallel. Or use a small incandescent night light bulb (4, 5, 6, or 7 watts). Those neon outlet testers won't detect bad connections or other problems and are only good for giving a rough indication. They are NOT the definitive test.

Why? Give me a good reason. As Leigh pointed out, anything that needs to be isolated will have a 2 wire cord so the ground isn't connected anyway.

That's fine until he maybe works on something other than a radio at some point, such as an amplifier, which my have a 3 prong cord.

He said that his outlet tester showed hot and neutral reversed. The only way the tester knows this is that the neutral side is connected to the hot wire (obviously) but also current flows from that now hot neutral side, through the lamp in the tester to the ground of the outlet. That right there says the ground is now able to complete a circuit that should not be. Add to that, when he pulled the outlet, things went to how they were supposed to be. Again, showing that the ground is able to complete a circuit.

Leigh wrote:

Neither terminal of the outlet is connected to ground at any point.

The ground going back to the breaker box and being connected to neutral at that point. Now that I read my post again, you are correct that it wouldn't have a direct connection to the hot or neutral prongs. I was thinking his problem as a whole. Obviously having a connected ground with the test equipment connected is not mixing.

The ground going back to the breaker box and being connected to neutral at that point. Now that I read my post again, you are correct that it wouldn't have a direct connection to the hot or neutral prongs. I was thinking his problem as a whole. Obviously having a connected ground with the test equipment connected is not mixing.

My comment referred to the iso secondary. It has two wires, neither of which is grounded.

You are correct regarding grounding of the primary neutral through the service box.

Here's one other thing to check, just to be sure of the isolation transformer. Take your multimeter, set it on ohms, and connect one lead to one of the input winding connections of the transformer, then connect the other meter lead to one of the winding connections on the other side of the transformer. You should get no reading. If you do get a reading, disconnect both leads from either the primary or the secondary (so that one or the other has both of it's lead disconnected) and check again. If you have a reading, then your transformer is internally shorted. If not, you have something in the case wired from one side to the other.

That's fine until he maybe works on something other than a radio at some point, such as an amplifier, which my have a 3 prong cord.

If it has a 3 wire cord, the AC line is already isolated from the chassis unless there is a defect or a previous "repairman" did something stupid. In either case you want the chassis grounded to protect yourself. Absolutely do ground the outlet.

If it has a 3 wire cord, the AC line is already isolated from the chassis unless there is a defect or a previous "repairman" did something stupid. In either case you want the chassis grounded to protect yourself. Absolutely do ground the outlet.

I disagree with you on that one. Without a ground connection, current flows out of the isolation transformer, through the device, and back as a closed loop. If there is a defect or botched repair as you pointed out, it's contained so to speak in that closed loop. The whole point of the isolation transformer is that if I'm in some way grounded, and I touch a hot part of the device, there cannot be a complete circuit. If I touched the chassis and a hot lead, then I'd get zapped. By grounding the device under test back to the wall, if I'm grounded, I'm also now connected to that chassis.

So in other words, my oscilloscope is sitting on the bench, not even turned on. I happen to have my hand against the case (it's metal and grounded) and I touch a hot lead in the chassis, I'll get shocked. In fact, go back and read the very first post in this thread, second to last paragraph. He measured between the outlet of the variac, and ground on a piece of test equipment and got 100 volts.

EDIT: Just to note that I think there is something more wrong with the variac in question here than just the ground wire being connected. But I still say that the ground is adding to it.

I would start by taking out the isolation transformer, and measuring the resistance between each of the output leads and each of the input leads. The resistance should be infinite. Any other value indicates either a defective transformer, or a different type of isolation transformer as Leigh noted above.

Then put the transformer back in the circuit and repeat the resistance measurement between the outlet and the prongs on the input cord. Again, should be infinite all around. If not, you probably have a wiring error somewhere. If you prefer, you can do this test first, and measure the iso transformer out of circuit if this test fails.

The resistance measurement is the definitive test for isolation. Once that passes, you can go back to measuring voltages. As noted above, if you get a voltage reading, try first adding a 10K resistor between the two test points and measuring the voltage across the resistor to eliminate any ghost voltages caused by the input impedance of the DMM.

As for grounding the 3rd hole on the outlet, it's not necessary, nor is it normally done. As noted, it could compromise isolation in some cases and thereby defeat the purpose of the isolation. Grounding the case of your home-built variac/iso, however, is fine, and is the preferred approach for safety reasons.

The schematic you drew shows the ac outlet ground connected to the chassis or cabinet. If that is the case you have connected the outlet ground to the ac inlet ground therefore loosing your isolation.

To expand on Lee's comment ---

Isolation in this context means that the secondary of the isolation transformer does not have any connection to ground (and therefore also no connection to the power line hot or neutral). Consequently you can externally connect either side of the secondary to power line ground without causing any fireworks, for example when connecting grounded test equipment to the circuit common of an AA5 radio that is powered by the secondary of the isolation transformer.

An isolation transformer enables you to determine what point in the circuit under test is grounded, rather than it being determined by the ground connection at the electric service entrance in your home.

The comments made by many folks about the function and testing of the isolation transformers are indeed correct. The line input ground and the isolation outlet ground could cause a problem however. As I recall, the issue came up when the "ground" of the test equipment was connected to the radio's chassis. As long as the radio had a 2-wire plug fed directly by the isolation transformer, there would be no problem - assuming the transformer is good. If, on the other hand, the radio/device did have a 3-wire plug, then the variac/iso case carrying earth ground could be a problem. Time to pull the covers off that transformer and do a little sniffing around.

I used to have an arc welder - rated at 400+ amps. Primary was 230 v and drew almost 50a . It was made from a WWII surplus transformer. I used it on the farm to weld 1/2" plate quite often - did a great job. Only problem was, the electrode voltage was about 130 volts. You had to be careful not to touch the the piece to be welded and the stick at the same time. A couple good jolts and you quickly learned to make sure your gloves didn't have any holes in 'em. . . .

Ok...I'm off the golf course and I 'think' my internet connection is somewhat stable (for the moment). I've enjoyed this debate but it's left me in a quandary about what to do. I think I should be able to connect the ground wire to the ground lug of the outlet, yet my observations tell me otherwise. Here is what I am going to do (tomorrow...golf = beer, so no ac work tonight).

I'm going to take the isolation transformer out of the cabinet and connect ac directly to the primary. I think I should read 120vac across the secondary, and 0vac across either secondary lead and the green ground wire on the power cord...right? They should be isolated, so there should be no relationship between them. If that is correct and that is what I measure, I'll put the transformer back into the cabinet, connect the green wire to the cabinet and do the same test.

Am I thinking right?

Thanks to all for this debate. While I'm frustrated and confused now I'm certain I will come out of this with a better understanding of how this all works.

Several people have suggested taking resistance readings...I've checked for continuity from every wire/terminal in this thing to the cabinet and found none and documented that earlier in this thread, so I think we're good there. The only way there can be some sort of short is if something goofy is happening while it's under power.